A self-switched biasing quadrature voltage-controlled oscillator (VCO) is presented. It is implemented by directly injecting the oscillation signal of one VCO core into the other VCO core through the divided tail current sources without additional active devices for coupling. The proposed coupling structure automatically switches the NMOS field-effect transistors used in VCO cores and current sources from strong inversion to accumulation. Since the deep switching of MOSFETs was reported to physically reduce flicker noise, the proposed quadrature VCO (QVCO) is expected to improve the phase noise performance, which is confirmed experimentally. The designed QVCO using 0.18- mum CMOS technology operates from 1.86 to 2.2 GHz with a 17% frequency tuning range. The measured phase noise is from - 129.1 to - 134.5 dBc/Hz at a 1-MHz offset, which is really close to ideal simulation results with the NMOS model disabling the flicker noise components. The average measured phase noise is 7.2 dB below the simulated one with a flicker noise model, which verifies the physical reduction of flicker noise by deep switching of the MOSFET. The phase noise figure-of-merit ranges from 179 to 185 over the entire tuning range. The QVCO dissipates 20 mA from a 1.8-V supply.